Field
The invention relates to wireless communications and, in particular, to positioning of a mobile device of a cellular communication system, for example.
Detailed Description of the Related Art
Observed Time Difference of Arrival (OTDOA) is a known positioning method based on measuring reference signals received by a mobile device from a plurality of fixed positioning nodes, e.g. access nodes of a wireless network. On the basis of the reference signals, a reference signal time difference observed by the mobile device with respect to the fixed positioning nodes may be calculated. The reference signal time difference may represent a difference in radio distances between the mobile device and a first access node and the mobile device and a second access node. At least two reference signal time difference values associated with different sets of access nodes may be used to estimate the location of the mobile device. In other words of saying, at least three measurements associated with difference access node may be needed for the estimation of the location of the mobile device.
Such positioning may be used for positioning the mobile device in locations where a global navigation satellite system (GNSS) such as Global Positioning System, Galileo, or GLONASS cannot provide a reliable positioning estimate.
For example, some modern cellular communication systems define a positioning protocol for positioning terminal devices of the system. An example of such a protocol is a LTE Positioning Protocol (LPP) of Long-Term Evolution cellular communication system. In the LPP, an OTDOA procedure works as follows: An evolved serving mobile location center (ESMLC) or a Secure User Plane Location Platform (SLP) requests, through an LPP layer, an OTDOA measurement. The request may be addressed to a terminal device and request the terminal device to perform a set of RSTD (Reference Signal Time Difference) measurements. Together with this request, the terminal device may receive assistance data. This assistance data may comprise a list of cells (eNodeBs) and PRS (Positioning Reference Signal) parameters of the cells. The terminal device may then perform the requested measurements. The measurements comprise estimating the exact time offsets between the PRS from different cells. Then it reports to the ESMLC these estimated time differences together with an estimate of the measurement quality or a measurement error. The ESMLC may then use these time difference estimates and the knowledge of the cells positions and transmit time offsets to estimate the position of the terminal device.
Measurement inaccuracies may degrade the performance of the conventional OTDOA algorithms. One cause of the inaccuracies may be multipath propagation of radio signals between an access node and the mobile device.